It is already known to use a laser beam for welding a plurality of workpieces together. To perform such laser welding, a laser beam is directed at the region of the interface between the workpieces to be connected with one another by a welded joint, and a relative movement is effected along the interface between the laser beam and the assembly of the workpieces. Usually, the intensity of the laser beam is sufficient for the laser beam to penetrate through the entire interface between the workpieces to make the weld extend from one side of the assembly of workpieces to the other.
Experience with this approach has shown that it has several disadvantages. For one, since it is desirable for the welded joint to exhibit a narrow width relative to the thickness of the workpieces being joined in order to minimize energy input, and since the ratio of weld width to thickness, generally speaking, remains approximately constant with increasing thickness of the workpieces at about 5:1 more, the amount of workpiece base material that is being heated to, or above, the melting temperature of such material may be excessive, especially when the thickness of the workpieces is large. Moreover, the quality of the root bead in thick weldments is often quite poor, resulting in a reduced quality of the welded joint as a whole. Furthermore, because of the existence of thermal stresses between the two oppositely facing sides of the workpiece assembly during the performance of the laser welding operation due to the one-sided application of the laser beam to the workpiece assembly, some warping may occur in the workpieces being joined, resulting in diminished quality of the welded workpiece assembly and/or requirements for significant post-weld machining.
It has also been proposed, particularly for situations in which workpieces of substantial thickness are to be welded together, to weld the workpiece assembly from one side first during an initial welding pass, and to subsequently complete the welded joint by welding the partially welded workpiece assembly from its other side during a second welding pass. While this approach results in a substantial reduction or total elimination of the problem of poor root bead quality, it merely transfers the problem area deeper into the welded joint. This is so because the subsequent laser beam does not carry a sufficient amount of energy for penetration through the workpieces to the other side of the assembly, so that this subsequent laser beam forms a blind weld, which characteristically results in aggravated plasma conditions and generation of voids, bubbles or similar welded joint porosity at the root of the second pass penetration. Also, at least the laser beam used during the second welding pass has to carry an amount of energy sufficient for penetration into the root portion of the welded region formed during the initial pass so as to provide an effective fusion zone tie between the separate regions of the welded joint. This not only results in an increased energy consumption relative to the theoretical minimum, but also aggravates the aforementioned plasma formation and venting problems brought about by operation in a blind hole and resulting in welded joint porosity.
Accordingly, it is a general object of the present invention to avoid the disadvantages of the prior art.
More particularly, it is an object of the present invention to provide a laser welding method which does not possess the disadvantages of the known methods of this kind.
Still another object of the present invention is so to develop the method of the type here under consideration as to reduce the energy consumption and improve the quality of the welded joint as compared to those obtained when using the heretofore known laser welding methods.
A concomitant object of the present invention is to devise a laser welding arrangement which is particularly suited for the performance of the above method.
It is yet another object of the present invention to design the laser welding arrangement of the above type in such a manner as to be relatively simple in construction, inexpensive to manufacture, easy to use, and yet reliable in operation.